U.S. patent number 4,611,159 [Application Number 06/637,219] was granted by the patent office on 1986-09-09 for ac motor control system.
This patent grant is currently assigned to Fanuc Ltd.. Invention is credited to Mitsuo Kurakake, Keiji Sakamoto.
United States Patent |
4,611,159 |
Kurakake , et al. |
September 9, 1986 |
AC motor control system
Abstract
There are provided a sensor (113) for sensing the velocity of an
AC motor (101), a sensor (112) for sensing an actual current
flowing into the AC motor (101), a power drive circuit for driving
the AC motor (101), and a control unit (108) for performing a
velocity loop computation to obtain a current command from an
offset velocity between a velocity command for the AC motor (101)
and the sensed actual velocity, and for performing a current loop
computation to obtain an offset current between the current command
and the sensed actual current. In the current loop computation
performed by the control unit (108), there is obtained a velocity
compensation signal by amplifying the sensed actual velocity by a
predetermined magnification, the command for the power drive
circuit is compensated by the velocity compensation signal, the
current loop computation is executed at a sampling period shorter
than that at which the velocity loop computation is executed, and,
at the time of the velocity loop computation, the actual current at
the relevant sampling instant is estimated from the current command
value of the previous sampling, and a current command is computed
from the estimated actual current and the offset current.
Inventors: |
Kurakake; Mitsuo (Hino,
JP), Sakamoto; Keiji (Hachioji, JP) |
Assignee: |
Fanuc Ltd. (Minamitsuru,
JP)
|
Family
ID: |
16523141 |
Appl.
No.: |
06/637,219 |
Filed: |
July 23, 1984 |
PCT
Filed: |
November 25, 1983 |
PCT No.: |
PCT/JP83/00421 |
371
Date: |
July 23, 1984 |
102(e)
Date: |
July 23, 1984 |
PCT
Pub. No.: |
WO84/02235 |
PCT
Pub. Date: |
June 07, 1984 |
Foreign Application Priority Data
|
|
|
|
|
Nov 25, 1982 [JP] |
|
|
57-206424 |
|
Current U.S.
Class: |
318/803;
318/811 |
Current CPC
Class: |
H02P
25/024 (20160201); H02P 23/0077 (20130101) |
Current International
Class: |
H02P
25/02 (20060101); H02P 23/00 (20060101); H02P
005/40 () |
Field of
Search: |
;318/811,803,807-810 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Le-Huy et al., "Microprocessor Control of a Current-Fed Synchronous
Motor Drive", Conference: Industry Applications Society IEEE-IAS
Meeting, Cleveland, OH, U.S.A. (Sep. 30-Oct. 4, 1979)..
|
Primary Examiner: Smith, Jr.; David
Attorney, Agent or Firm: Staas & Halsey
Claims
We claim:
1. An AC motor control system comprising:
(a) velocity sensor means for sensing the velocity of an AC
motor;
(b) current sensor means for sensing an actual current flowing into
said AC motor;
(c) power drive circuit means for receiving a drive command and for
driving said AC motor in accordance with said drive command;
and
(d) control means for performing a velocity loop computation at a
first sampling period to obtain a current command from an offset
velocity between a velocity command for said AC motor and said
sensed actual velocity, and for performing a current loop
computation to obtain an offset current between said current
command and said sensed actual current, in said current loop
computation, there is obtained a velocity compensation signal by
amplifying said sensed actual velocity by a predetermined
magnification, and the drive command for said power drive circuit
is compensated by said velocity compensation signal, said current
loop computation is executed at a second sampling period being
shorter than said first sampling period, and, at the time of the
velocity loop computation, the actual current at the relevant
sampling instant is estimated from the current command value of the
previous second sampling period, and the current command is
computed from the estimated actual current and said offset
current.
2. An AC motor control system according to claim 1, characterized
in that said loop computation and said current loop computation are
performed by a single processor.
3. An AC motor control system according to claim 1, characterized
in that said velocity loop computation and said current loop
computation are performed by separate computation control units for
each computation.
Description
DESCRIPTION
Background of the Invention
This invention relates to an AC control system in which a velocity
loop computation and a current loop computation for an AC motor are
performed by a microprocessor to control the AC motor. More
particularly, the invention relates to an AC motor control system
capable of enhancing the response characteristics of the velocity
and current loops.
An arithmetic circuit such as a microprocessor has recently come to
be employed for servo-controlling an AC motor. It is required that
the microprocessor execute at least a velocity loop computation, in
which a current command is computed from an offset velocity between
a commanded velocity and the actual velocity of the AC motor, and a
current loop computation, in which a command for application to the
current drive circuit of a servomotor is computed based on a
difference between a current command and the armature current of
the servomotor.
In order to obtain a desirable servomotor operating characteristic,
it is required that the response characteristic of the current loop
be quicker than that of the velocity loop. Since there is
interference between current and velocity ascribable to a back
electromotive force in a servomotor, the velocity loop and current
loop computations cannot be rendered independent of each other and
both computations must be executed at a predetermined sampling
period. The result is a burden upon the microprocessor in terms of
processing time.
Moreover, even if the velocity computation is executed at a period
which is longer than that at which the current loop computation is
executed, the computation for obtaining the effective current
requires a long period of time when performed at the time of the
velocity loop computation. A problem that results is a
deterioration in the response of the velocity loop.
Summary of the Invention
An object of the present invention is to provide an AC motor
control system capable of enhancing the response characteristic of
a current loop and, moreover, of a velocity loop.
An AC motor is controlled by providing a sensor for sensing the
velocity of the AC motor, a sensor for sensing an actual current
flowing into the AC motor, a power drive circuit for the AC motor,
and a control unit for performing a velocity loop computation to
derive a current command from an offset velocity between a velocity
command for the AC motor and the sensed actual velocity, and for
performing a current loop computation to obtain an offset current
between the current command and the sensed actual current, the
offset current obtained by the control unit being applied to the
power drive circuit to control the AC motor. In the current loop
computation, a velocity compensation signal is obtained by
amplifying the sensed actual velocity by a predetermined
magnification, the command for the power drive circuit is
compensated by the velocity compensation signal, and the current
loop computation is executed at a sampling period shorter than that
at which the velocity loop computation is executed. At the time of
a velocity loop computation, the actual current at the relevant
sampling instant is estimated from the current command value of the
previous sampling, and a current command is computed from the
estimated actual current and the offset current.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a view showing the construction of a synchronous
motor;
FIG. 2 is a block diagram of conventional servo control;
FIG. 3 is a view for describing velocity and current on the basis
of the conventional control system;
FIG. 4 is a block diagram of servo control according to the present
invention;
FIG. 5 is a circuit diagram of an embodiment of the present
invention;
FIG. 6 is a view showing the construction of a principal portion of
FIG. 5;
FIG. 7 is a view for describing the operation of the arrangement
shown in FIG. 6; and
FIGS. 8 and 9 are views for describing operation according to the
present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The present invention will now be described in detail in accordance
with the drawings.
A synchronous motor may serve as a servomotor which requires a
current loop having a quick response characteristic.
In a synchronous motor of this kind, it is necessary that torque be
controlled so as to be constant. To this end, there has been
developed a technique in which control is exercised in such a
manner that a current of the same phase as an electromotive force
induced by the rotor is caused to flow into the windings of the
armature, which serves as the stator. This technique will now be
described using the drawing of FIG. 1, which shows the construction
of a synchronous motor. The magnetic flux density B at a position
displaced by .theta. degrees from the q axis of the magnetic field
generated by a rotor 1, namely a permanent magnet, is given by the
following:
The magnetic flux .phi. interlinked with the a winding of a stator
2 shown in FIG. 1 is expressed as follows:
where .phi..sub.m represents the magnetic flux on the q axis of the
rotor 1.
Accordingly, the electromotive force e.sub.1 induced in the a
winding is expressed as follows: ##EQU1## (where
.theta.=P.theta.m=P.multidot..omega..sub.m .multidot.t).
Likewise, the electromotive forces e.sub.2, e.sub.3 induced in the
b and c windings of the stator 2, which are disposed at angles of
1/3.pi. and 2/3.pi. relative to the a winding, respectively, are
expressed by the following:
If we let the currents flowing in the armature windings a, b, c be
i.sub.1, i.sub.2, i.sub.3, respectively, then the output torque T
of such a three-phase synchronous motor will be expressed by the
following:
Therefore, substituting Eqs. (3), (4) and (5) into Eq. (6), we
have:
To render the torque T constant, it should be so arranged that T is
independent of the angle .theta.. Therefore, if the following
relations hold, namely: ##EQU2## where I is the current amplitude,
then the torque T of Eq. (7) may be written as follows: ##EQU3##
Thus, the torque T is constant, being independent of the rotational
orientation of the rotor 1.
To carry out such control, it is necessary to detect the rotor
angle of the synchronous motor and regulate each of the armature
current values in accordance therewith.
However, if the current flowing through each armature winding is
delayed by .phi..sub.o from the ideal value, then the currents
i.sub.1, i.sub.2, i.sub.3 of the respective armature windings will
take on the form:
In consequence, the output torque T will take on the form:
Thus, the torque will decrease in value.
Thus, in order to effect control to render the torque of a
synchronous motor constant, it is necessary to improve the actual
current response with respect to the current command. Specifically,
as shown in the block diagram of FIG. 2 illustrating a conventional
control circuit for a synchronous motor, the actual rotational
velocity v of a synchronous motor 101 is detected, the difference
between v and a commanded velocity VCMD is found by an arithmetic
unit 105, the velocity difference obtained is converted into a
current command I by a velocity loop computing circuit 106,
thereafter the difference between the current command I and the
actual current i flowing into the synchronous motor 1 is computed
by an arithmetic unit 110, the current difference is operated upon
by a current loop computing circuit 113, and the output of the
computing circuit 113 is power amplified by a pulse width
modulator/inverter circuit 115, the output of the circuit 115 being
applied to the synchronous motor 101.
To execute the foregoing by a microprocessor, the operation
performed by the circuitry from the arithmetic unit 105 to the
current loop arithmetic unit 113 should be executed by computer
processing. The processing is required to be carried out at a
sampling period dependent upon the response characteristic of the
current loop. However, since there is interference between current
and velocity ascribable to a reverse electromotive force in the
servomotor, the velocity loop and current loop computations cannot
be rendered independent of each other and both computations must be
executed at a predetermined sampling period. The result is a burden
upon the microprocessor in terms of processing time.
Therefore, according to the present invention, it is arranged so
that the current loop can be operated independently of the velocity
loop, and so that the period of the current loop computation is
made shorter than that of the velocity loop computation. Stated in
reverse, the period of the velocity loop computation is lenthened,
the microprocessor load is lightened, and the period of the current
loop computation is shortened.
Further, according to the present invention, in order to enhance
the response of the velocity loop computation the period thereof if
lengthened, an armature current component is fed back to the
velocity loop. Moreover, this armature current component is
predicted from the current command of the previous sampling to
prevent an increase in the processing time of the
microprocessor.
FIG. 4 is a block diagram illustrating an embodiment of the present
invention. Portions similar to those shown in FIG. 2 are designated
by like reference characters. Numeral 120 denotes an adder, and 121
a multiplier, these elements constituting velocity feedback. MP1
denotes a first microprocessor for performing the operations of the
arithmetic circuit 105 and velocity loop arithmetic unit 106 by
computer processing at a period T.sub.2. MP2 denotes a second
microprocessor for performing the operations of the arithmetic
circuit 110 and current loop arithmetic unit 113 by computer
processing at a period T.sub.1. The periods T.sub.1, T.sub.2 of the
microprocessors MP1, MP2 are related as follows:
where n is an integer of two or more.
The first microprocessor MP1 also performs IP control by executing
the velocity loop computation, and the output u(k) of the
microprocessor MP1 is given by the following equation: ##EQU4##
where v(i) represents the actual rotational velocity, r(i) the
commanded velocity, and i(k) the effective armature current.
The effective armature current i(k) is expressed by the following
equation using the actual phase currents I.sub.av, I.sub.aw,
I.sub.au of the synchronous motor 101: ##EQU5##
However, since execution time for a multiplication instruction is
longer than that required for other instructions as far as a
microprocessor is concerned, sensing the phase currents and
performing this computation increases the load on the
microprocessor MP1. Therefore, the output u(k-1) at the time of the
previous sampling is assumed to be the effective armature current
at timk k:
That is, when the output u(k-1) from the microprocessor MP1 is
delivered at time (k-1) to the current loop (microprocessor MP2),
the current loop exercises control at the period T.sub.1 in such a
manner that the effective current attains the value u(k-1) by the
next sampling instant (k) of the velocity loop. In other words, the
current loop is designed with a set proportional integration
coefficient, described below, in such a manner that the actual
phase current is set to the compound output u of the velocity loop
during the sampling period T.sub.2. With such an arrangement, the
effective current at the sampling instant k can be assumed to be
u(k-1). Accordingly, substituted Eq. (14) into Eq. (12) gives us:
##EQU6##
As a result of the microprocessor MP1 performing the operation of
Eq. (15) every point T.sub.2, it becomes possible to carry out IP
control by complete feedback control of the object controlled,
thereby stabilizing the system and introducing an integration
characteristic with respect to the error between a command and a
controlled quantity to further improve the response
characteristic.
Further, in FIG. 4, the microprocessor MP2, which includes the
current loop arithmetic unit, is provided with velocity feedback
(the multiplier 121 and adder 120) to cancel the actual velocity
dependence of the current loop and to render the operation of the
current loop independent of the operation of the velocity loop.
Specifically, if the synchronous motor 101 is expressed in terms of
a transfer function, as shown in FIG. 2, the current loop includes
feedback from the velocity v, which is attributed to the back
electromotive force constant Ke of the motor. TR represents load
torque, and La, Ra, Kt, J denote transfer constants. This velocity
feedback has an effect upon the actual current. At high velocity,
the current loop is influenced by the velocity v, resulting in
diminished actual current response.
More specifically, let us consider acceleration. As shown in FIG.
3, in a situation where velocity feedback is negligible, velocity v
and actual current i make ideal transitions with respect to time t,
as shown by the dashed lines. Due to velocity feedback, however,
the actual current i is influenced by the velocity v, as shown by
the solid line. The result is a greater current value and prolonged
acceleration time.
In FIG. 2, a differential equation involving actual velocity v and
actual current i, in which load torque is negligible, may be
written as follows: ##EQU7## The foregoing will be explained below
in terms of a discrete value system considering implementation by a
microprocessor.
Rewriting Eq. (10) in a discrete value system at a sampling period
T will give us the following:
where u(k) represents the output of the current loop computing
circuit 113.
It will be appreciated from Eq. (18) that eliminating the velocity
term v(k) will render the current i(k+1) independent of
velocity.
Therefore, the arrangement is such that velocity feedback is
applied to the current loop, and the inherent velocity feedback of
the synchronous motor is cancelled. If we assume that the
multiplier 121 has a transfer constant kf and that this velocity
feedback is applied to Eq. (12), then Eq. (18) may be written:
Therefore, if a selection is made such that:
then Eq. (19) will reduce to:
so that the actual current i(k+1) will be independent of the
velocity v.
Accordingly, the characteristic of the current loop can be
controlled independently of the velocity of the synchronous motor
and, hence, there will be no deterioration in the response of the
current loop even at high speeds.
FIG. 5 is a circuit diagram of an embodiment of the present
invention, in which the velocity loop and current loop computations
are executed by a single microcomputer.
In the Figure, numeral 101 denotes a synchronous motor of revolving
field type. Numeral 108 denotes a computation control unit which,
by a computing operation based on a control program, performs the
operations of the arithmetic circuit 105, velocity loop computing
circuit 106, arithmetic circuit 110, current loop computing circuit
113 and adder 120 of FIG. 4. The computation control unit 108
comprises a processor 108a for performing arithmetic operations in
accordance with a motor control program, a program memory 108b
storing the motor control program, a data memory 108c for storing
data, an input/output port 108d for receiving commands from an
external unit such as an NC unit, a digital-to-analog (DA)
converter 108e for applying an analog current command to a
pulse-width modulation circuit 114, an analog-to-digital (AD)
converter 108f which receive phase currents I.sub.au, I.sub.av,
I.sub.aw from current transformers 112U, 112V, 112W for converting
these into digital values, a counter 108g in which a position code
indicating the rotational position .alpha. of the field pole of the
synchronous motor 101 is initially loaded from a pulse coder 112,
the counter thereafter counting rotation pulses P1, P2 generated by
the pulse coder 112 whenever the synchronous motor 101 rotates
through a predetermined angle, and an address/data bus 108h for
interconnecting the foregoing components. The pulse coder 113
generates a position code indicating the position of the field pole
of the synchronous motor 101, as well as rotation pulses produced
whenever the motor 101 rotates through a predetermined angle.
Numeral 114 denotes a pulse width modulation circuit, and 115 an
inverter circuit controlled by the output signal of the pulse-width
modulation circuit. Numeral 116 denotes a three-phase A.C. power
supply, and 117 a well-known rectifier circuit comprising a group
of diodes 117a and a capacitor 117b for converting the three-phase
alternating current into direct current. As illustrated in FIG. 6,
the pulse width modulation circuit 114 comprises a sawtooth
generating circuit for generating a sawtooth waveform STS,
comparators COM.sub.U, COM.sub.V, COM.sub.W, NOT gates NOT.sub.1
through NOT.sub.3, and drivers DV.sub.1 through DV.sub.6. The
inverter INV includes six power transistors Q.sub.1 through Q.sub.6
and six diodes D.sub.1 through D.sub.6. The comparators COM.sub.U,
COM.sub.V, COM.sub.W of the pulse width modulation circuit PWM
compare the sawtooth signal STS with the amplitudes of the
three-phase alternating current signals I.sub.u, I.sub.v, I.sub.w,
respectively, and produce a "1" output when I.sub.u, I.sub.v or
I.sub.w is greater than the value of STS, or a "0" output when
I.sub.u, I.sub.v or I.sub.w is smaller. Thus, with respect to
i.sub.u, the comparator COM.sub.U produces the current command
i.sub.uc shown in FIG. 7. More specifically, pulse-width modulated
three-phase current commands i.sub.uc, i.sub.vc, i.sub.wc dependent
upon the amplitudes of i.sub.u, i.sub.v, i.sub.w are produced.
These three-phase current commands i.sub.u, i.sub.v, i.sub.w are
delivered as inverter drive signals SQ.sub.1 through SQ.sub.6 via
NOT gates NOT.sub.1 through NOT.sub.3 and drivers DV.sub.1 through
DV.sub.6, and are applied as input signals to the inverter 115. The
inverter drive signals SQ.sub.1 through SQ.sub.6 input to the
inverter 115 are applied to the bases of the power transistors
Q.sub.1 through Q.sub.6, respectively, thereby controlling the
on/off action of the power transistors Q1 through Q6 to supply the
synchronous motor 101 with a three-phase current.
Discussed next will be the operation of the arrangement of FIG. 5
in a case where the velocity command is elevated while the
synchronous motor 101 is rotating at a certain velocity. The
counter 108g is loaded with a position code immediately prior to
the start of rotation of the synchronous motor 101, and the counter
counts the rotation pulses P.sub.1, P.sub.2 which are generated as
the synchronous motor 101 rotates. Accordingly, the counter 108g
indicates the rotational position of the field pole of synchronous
motor 101 at all times. Since the rotational pulses P.sub.1,
P.sub.2 will be proportional to the velocity of the synchronous
motor 101, the amount increase in the count in the counter 108g
over a prescribed time interval will correspond to the rotational
velocity of the synchronous motor 101.
(1) First, to rotate the synchronous motor 101 at a desired
rotational velocity V.sub.c, the input/output port 108d is supplied
with a velocity command VCMD from an external unit such as an NC
unit. This command is transmitted to the processor 108a via the bus
108h. Next, the processor 108a reads the value of the count in
counter 108g via the bus 108h, computes the difference between this
value and that read previously, and divides the difference by the
sampling interval T.sub.2 to compute the actual velocity Va (actual
velocity sensing step). (2) Further, the processor 108a computes a
velocity error ER, which is the difference between the velocity
command VCMD and the actual velocity Va, and performs the operation
indicated by the foregoing Eq. (15). Specifically, ##EQU8## and
u(k-1) used at time (k-1), which is the previous sampling instant,
are stored in the data memory 108c beforehand. Then, the following
is computed: ##EQU9## where ER=v(k)-r(k) followed by computation of
the following:
Then, s(k-1) in the data memory 108c is updated to s(k), and u(k-1)
is updated to u(k) to prepare for the next computation. It should
be noted that the results obtained by performing the computation of
Eq. (22) correspond to the amplitude of the armature current.
Specifically, when the load varies or the velocity command changes,
the velocity error ER becomes greater, as does the current command
amplitude I.sub.s correspondingly. The increased amplitude I.sub.s
results in the production of a greater torque, which brings the
actual rotational velocity of the motor into conformity with the
commanded velocity. The amplitude command I.sub.s is obtained (IP
control computation step).
The foregoing is a velocity loop computation step, which is
performed at every sampling period T.sub.2, as shown in FIG. 8.
(3) Next, based on the value of the count in counter 108g, the
processor 108a retrieves, from a table stored in the data memory
108c, the digital value of sin .alpha. indicating the rotational
position .alpha. of the field pole of synchronous motor 101, as
well as the digital value of sin (.alpha.+2.pi./3) indicating the
rotational position .alpha.+2.pi./3. Using these values, the
processor 108a computes the three phase current commands I.sub.u,
I.sub.v, I.sub.w from the following equations: ##EQU10##
(4) Next, the processor 108a reads, via the bus 108h, the actual
currents obtained by a digital conversion applied by the AC
converter 108f to the actual phase currents I.sub.av, I.sub.aw,
I.sub.au obtained from the current transformers 112U, 112V, 112W,
respectively, computes the error between the three phase current
command I.sub.u, I.sub.v, I.sub.w and the actual phase currents
I.sub.av, I.sub.aw, I.sub.au, and performs the proportional
integration operations given by the following equations to obtain
the command values i.sub.u, i.sub.v, i.sub.w for application to the
DA converter 108e: ##EQU11##
Thus, when the processor 108a produces the current command I.sub.s
(k-1) at the instant k-1, in order for the current loop to exercise
control in such a manner that the effective current of the AC motor
attains the value I.sub.s (k-1) by the next sampling instant (k) of
the velocity loop, or in other words, in order for the current loop
to carry out control at the sampling period T.sub.1 in such a
manner that the effective current of the AC motor is set to the
current command I.sub.s of the velocity loop during the sampling
period T.sub.2, the gains K.sub.1 ', K.sub.2 ' in Eq. (24) are
adjusted. The described control state will thus be realized.
(5) Next, the processor 108a multiplies the actual velocity Va,
which was obtained in the foregoing velocity loop computation step,
by the coefficient kf, obtains a velocity compensation output VCO,
and subtracts this from the three-phase AC signals i.sub.u,
i.sub.v, i.sub.w, thereby obtaining compensated three-phase AC
signals signals i.sub.u, i.sub.v, i.sub.w. The foregoing is a
current loop computation step, which is performed at every sampling
period T.sub.1 shown in FIG. 8.
The processor 108a sends the compensated three-phase AC signals
i.sub.u, i.sub.v, i.sub.w thus obtained to the DA converter 108e
via the bus 108h. These three-phase AC signals i.sub.u, i.sub.v,
i.sub.w are thus converted into analog quantities which are
delivered to the pulse-width modulating circuit 114. Thereafter,
and in similar fashion, the synchronous motor 101 is supplied with
three-phase current from the inverter 115.
The processor 108a performs the velocity loop computation at every
sampling period T.sub.2, and performs the current loop computation
at every sampling period T.sub.1. In the example of FIG. 8, the
current loop computation is performed four times for each velocity
loop computation. As shown in FIG. 9, the current command base on
the velocity loop computation varies in a step-like manner at the
period T.sub.2. By contrast, since the current loop computation is
performed four times, the requirement that the current loop have a
quick response characteristics can be satisfied if the gain of Eq.
(24) is adjusted and the current response is set with respect to
the command during T.sub.2. The processor 108a controls the
synchronous motor 101 by executing the above-described computations
in accordance with the control program in the program memory 108b
at the sampling periods T.sub.1, T.sub.2, whereby the synchronous
motor 101 will eventually be rotated at the commanded velocity.
In the embodiment of FIG. 5, the velocity and current loop
computations are performed by a single computation control unit.
However, separate computation control units can be provided for the
velocity and current loops, as shown in FIG. 4.
Further, an arrangement may be adopted in which the DA converter
108e in FIG. 5 is constituted by a counter and the processor 108a
delivers the digital three-phase AC signal to this counter via the
bus 108h, with the counter producing a pulse-width modulated
output. In a case where pulse-width modulation is performed, an
arrangement may be adopted as disclosed in the specification of
Japanese Patent Application Laid-Open No. 53-48449, wherein a
digital integrated circuit equipped with a clock generating circuit
is used to drive an up/down counter circuit, and a programmable
read-only memory responds to the up/down counter by generating a
digital output signal which is proportional to a pulse-width
modulated drive signal required for the power stage. Thus, a DC
voltage may be digitally converted directly into an AC output
waveform without using an AC reference waveform. In such case, the
pulse-width modulation signal is delivered to a base drive circuit,
so that an inverter can be controlled by this signal.
According to the present invention as described above, in
performing a velocity loop computation and a current loop
computation by an arithmetic circuit, the period of the current
loop computation is shorted and the period of the velocity loop
computation is lengthened. Therefore, an advantage which results is
that the response characteristic of the current loop can be
improved within the processing capability of the arithmetic
circuit. In addition, the velocity and current loops can be
rendered independent of each other. In performing the velocity loop
computation, the amplitude command at the relevant sampling instant
is corrected by the amplitude command value of the previous
sampling, so that IP control is possible without computing the
effective current at the relevant sampling instant. The resulting
advantage is that the response characteristic of the velocity loop
can be improved. This makes it possible to shorten the period of
the current loop computation to an even greater extent.
While the present invention has been described in accordance with
an embodiment thereof, the present invention is not limited to said
embodiment but can be modified in various ways in accordance with
the gist thereof without departing from the scope of the
invention.
According to the present invention, in a system for controlling an
AC motor by using a microprocessor to perform the velocity and
current loop computations of the AC motor, the velocity and current
loop response characteristics can be improved. The present
invention therefore is well-suited for application to the field of
AC motor control.
* * * * *